Generally, a semiconductor device includes transistor gates formed on a semiconductor substrate and source/drain regions formed on the semiconductor substrate on opposed sides of the gate. As semiconductor devices become more highly integrated, the size of the transistor gate continues to shrink and the source region becomes closer to the drain region. This proximity produces an undesirable short-channel effect. In order to minimize this short-channel effect, spacers are conventionally formed along the sidewalls of the transistor gates. The presence of the spacers increases the distance between the source and drain regions in a particular transistor and also spaces each of the source and drain regions from the channel because the ion implantation process used to form the source and drain regions in the substrate does not implant dopant impurities through the spacers. The spacer width therefore determines the spacing between the source and drain regions and between the channel and each of the source/drain regions. Many functional device characteristics and parameters such as the transistor saturation current, Isat, are highly dependent upon the spatial arrangement of the source, drain and channel, and therefore the spacer width. It is therefore important to accurately produce spacers having desired widths. This becomes even more important when device sizes are scaled down and the associated spacers have correspondingly reduced widths.
Spacers are conventionally produced by forming at least one dielectric film over a transistor gate then performing an anisotropic dry etch to form spacers along the vertical sidewalls of the transistor gates. A growing trend is to form spacers from multiple dielectric films formed over the gate structure. Silicon nitride is a commonly favored spacer material. In particular, it has become increasingly popular to form silicon nitride spacers by forming a film stack of an oxide layer over a silicon nitride layer, then etching the film stack to form a composite spacer having portions of the silicon nitride film and the oxide film. A dry etching process is then used to selectively remove the oxide portion of this composite spacer, leaving a silicon nitride spacer having a width that determines the spatial arrangement of the source, drain and channel and therefore various device characteristics and parameters such as the saturation current. It is therefore critical to accurately control the width of such silicon nitride spacers.
It is difficult to control the width of the silicon nitride spacers, however, by controlling the plasma etch processes used to form the silicon nitride spacers, i.e., the etch process used to form the composite spacer, the etch process used to form the final silicon nitride spacer and the various overetch steps, because the plasma etching processes etch very rapidly and use etching times of extremely short duration.
It would therefore be desirable to provide an apparatus and method to accurately produce spacers, particularly silicon nitride spacers, that have desired spacer widths.